feat(update-checker): give manual checks visible, timestamped feedback

- Manual check finds no newer version: status message becomes "Redline X.Y.Z is up to date, checked HH:MM Dubai"
- Manual check stages a newer version: status message becomes "Update to X.Y.Z is ready"
- Automatic (scheduled) checks keep original silent behaviour when up to date
- Add snapshot-only seams for testing: snapshotPreviousVersionOverride and snapshotUsesPreviousVersionOverride

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ZiTXPbPCSjzPVsfoweAbp

Sources/Shotdeck/UpdateChecker.swift

@@ -2,6 +2,7 @@ import AppKit
 import CryptoKit
 import Foundation
 import Security
+import ShotdeckCore
 
 /// Built-in updater. Checks an appcast, stages a verified payload, and installs
 /// only when the user clicks the menu row — never automatically.
@@ -31,6 +32,10 @@ final class UpdateChecker {
     private var repeatingTimer: Timer?
     private var firstCheckTask: Task<Void, Never>?
     private var stagingDirectory: URL?
+    /// Snapshot-only override for previousVersion; when snapshotUsesPreviousVersionOverride is true,
+    /// this value (including nil) is returned instead of checking the file system.
+    var snapshotPreviousVersionOverride: String?
+    var snapshotUsesPreviousVersionOverride: Bool = false
 
     init() {
         let config = URLSessionConfiguration.ephemeral
@@ -85,7 +90,12 @@ final class UpdateChecker {
 
         guard Self.isNewer(appcast.version, than: Self.currentVersion()) else {
             clearOffer()
-            statusMessage = manual ? "Redline \(Self.currentVersion()) is up to date." : nil
+            if manual {
+                let timestamp = DubaiTime.checkTime(lastCheckedAt ?? Date())
+                statusMessage = "Redline \(Self.currentVersion()) is up to date, checked \(timestamp)"
+            } else {
+                statusMessage = nil
+            }
             onChecked?()
             return
         }
@@ -93,7 +103,7 @@ final class UpdateChecker {
         do {
             try await downloadAndStage(appcast)
             availableUpdate = (version: appcast.version, notes: appcast.notes ?? "")
-            statusMessage = nil
+            statusMessage = manual ? "Update to \(appcast.version) is ready" : nil
         } catch UpdateCheckError.checksumMismatch {
             discardStaging()
             availableUpdate = nil
@@ -226,8 +236,11 @@ final class UpdateChecker {
     }
 
     /// The version recorded in `Redline.app.previous`'s Info.plist, or nil when no
-    /// rollback copy exists.
+    /// rollback copy exists. Respects the snapshot-only override for panel rendering.
     func previousVersion(target: URL = UpdateChecker.defaultInstallTarget) -> String? {
+        if snapshotUsesPreviousVersionOverride {
+            return snapshotPreviousVersionOverride
+        }
         let previousURL = target.deletingLastPathComponent().appendingPathComponent("Redline.app.previous")
         let plistURL = previousURL.appendingPathComponent("Contents/Info.plist")
         guard let plist = NSDictionary(contentsOf: plistURL) as? [String: Any] else { return nil }